Definitions

stratigraphy

stratigraphy

[struh-tig-ruh-fee]
stratigraphy, branch of geology specifically concerned with the arrangement of layered rocks (see stratification). Stratigraphy is based on the law of superposition, which states that in a normal sequence of rock layers the youngest is on top and the oldest on the bottom. Local sequences are studied, and after considering such factors as the average rate of deposition of the different rocks, their composition, the width and extent of the strata, the fossils contained, and the periods of uplift and erosion, the geological history of the sequence is reconstructed. These sequences are then correlated to those of similar age in other regions with the ultimate aim of establishing a consistent geochronology for the entire earth. Statigraphy is therefore important in the relative dating of all types of rock. In areas where the strata have undergone folding, faulting, and erosion, stratigraphic techniques are used to determine their correct sequence. The principle of included fragments in stratigraphy states that any rock fragment included in another rock must be older than the surrounding rock. Fossils have been the most important means of correlation because, as a result of evolution, rock strata of approximately equal age exhibit similar flora and fauna. Dating and correlation of stratified rocks by means of fossils is called stratigraphic paleontology. See also dating.

See B. Kummel, History of the Earth (1961); E. W. Spencer, Basic Concepts of Historical Geology (1962); R. K. Matthews, Dynamic Stratigraphy (1974); P. C. Cattermole and P. Moore, The Story of the Earth (1985).

Scientific discipline concerned with describing rock successions and interpreting them in terms of a general time scale. It provides a basis for historical geology, and its principles and methods are applied in such fields as petroleum geology and archaeology. Stratigraphic studies deal primarily with sedimentary rocks but may also encompass layered igneous rocks (e.g., those resulting from successive lava flows) or metamorphic rocks formed either from such extrusive igneous material or from sedimentary rocks.

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In chronostratigraphy, a stage is a succession of rock strata laid down in an single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.

Rock series are divided into stages, just as geological epochs are divided into ages. Stages can be divided into smaller stratigraphic units called chronozones. (See chart at right for full terminology hierarchy.)

The term faunal stage is sometimes used, referring to the fact that the same fauna (animals) are found throughout the layer (by definition).

Defining

Stages are primarily defined by a consistent set of fossils (biostratigraphy) or a consistent magnetic polarity (see paleomagnetism) in the rock. Usually one or more index fossils that are common, found worldwide, easily recognized, and limited to a single, or at most a few, stages are used to define the stage's bottom.

Thus, for example, in the (still used) local North American subdivision paleontologist finding fragments of the trilobite Olenellus would identify the beds as being from the Waucoban Stage whereas fragments of a later trilobite such as Elrathia would identify the stage as Albertan.

Stages were very important in the 19th and early 20th century as they were the major tool available for dating rock beds until the development of seismology and radioactive dating in the second half of the 20th Century. Microscopic analysis of the rock (petrology) is also sometimes useful in confirming that a given segment of rock is from a particular age.

Originally, faunal stages were only defined regionally; however as additional stratigraphic tools, and especially geochonological ones, were developed, stages were defined over broader and broader areas. More recently, the adjective "faunal" has been dropped as regional and global correlations of rock sequences have become relatively certain and there is less need for faunal labels to define the age of formations. A tendency developed to use European and, to a lesser extent, Asian, stage names for the same time period world wide, even though the faunas in other regions often had little in common with the stage as originally defined.

International standardization

Boundaries and names are established by the International Commission on Stratigraphy (ICS) of the International Union of Geological Sciences. As of 2008, the ICS is nearly finished a task begun in 1974, subdividing the Phanerozoic eonothem into internationally accepted stages using two types of benchmark. For younger stages, a Global Boundary Stratotype Section and Point (GSSP), a physical outcrop clearly demonstrates the boundary. For for older stages, a Global Standard Stratigraphic Age (GSSA) is an absolute date. The benchmarks will give a much greater certainty that results can be compared with confidence in the date determinations, and such results will have farther scope than any evaluation based solely on local knowledge and conditions.

In many regions around the world local subdivisions and classification criteria are still used along with the newer internationally coordinated uniform system, but once the research establishes a more complete international system, it is expected that local systems will be abandoned.

Stages and lithostratigraphy

Stages can include many lithostratigraphic units (for example formations, beds, members, etc.) of differing rock types that were being laid down in different environments at the same time. In the same way, a lithostratigraphic unit can include a number of stages or parts of them.

See also

Notes and references

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